The present invention concerns apparatus for making timing adjustments to sampled data signals which have been resampled, in particular, to apparatus for implementing subcarrier-to-horizontal (SCH) and video-to-horizontal timing adjustments in a digital video signal resampling system.
An international standard (known as SMPTE 125M) is currently being developed under the auspices of the Society of Motion Picture and Television Engineers (SMPTE). This standard for representing component video signals is derived from the CCIR601 standard developed by the International Consultative Committee for Radio-communications (CCIR). Under these standards, video information is represented by luminance (Y) and chrominance (C) components. The luminance signal which represents image brightness has a sample rate of 13.5 MHz, while the chrominance component consists of two interleaved color-difference signals, CR and CB, each of which have a sampling rate of 6.75 MHz. Under the 125M standard, samples of the luminance signal are multiplexed with samples of the two color-difference signals to form a sequence Y, CR, Y, CB, Y, CR . . . . This multiplexed signal has a sampling rate of 27 MHz.
The SMPTE 244M signal standard for North America is a digitized version of the NTSC video signal standard. This signal has a sample rate of 14.31818 (hereinafter 14.3) MHz and represents a base band composite video signal having a luminance signal component and two color-difference signal components which modulate respective quadrature-phase related subcarrier signals.
A system which converts 125M signals to 244M signals is described in U.S. Pat. No. 5,057,911 entitled, SYSTEM AND METHOD FOR CONVERSION OF DIGITAL VIDEO SIGNALS, which is hereby incorporated by reference for its teachings on video signal conversion. The system described in this patent uses a single interpolation filter to resample the luminance and chrominance components signals, to modulate the chrominance subcarrier signal by the chrominance signals and to combine the modulated chrominance signal with the luminance signal to generate the 244M composite video signal.
The first step in this conversion process is to demultiplex the 125M signal to generate separate Y and C component signals, where the C signal consists of interleaved samples of the CR and CB signals.
The demultiplexed Y signal has a sample rate of 13.5 MHz while the composite 224M signal has a sample rate of 14.3 MHz. Thus, for every 33 samples of the 125M Y signal, there are 35 samples of the 244M signal. The 125M C signals have a sampling rate of 6.75 MHz. Thus, for every 33 125M component chrominance sample sets (CR, CB) there are 70 244M composite samples. The system disclosed in the above-referenced patent resamples the component 125M signals to produce the composite 244M signal by using 35 interpolation filters for the signal Y, and 70 interpolation filters for the signal C. The 35 Y interpolation filters define 35 interstitial sample positions in the 125M Y sample interval while the 70 C interpolation filters define 35 interstitial sample positions between successive ones of the CR and CB samples, respectively. These filters are applied in a predetermined sequence to convert 66 samples of the 125M signal (66 Y and 33 CR,CB pairs) into 70 samples of the Y and C component signals which are then combined to form the 244M composite signal.
In addition to the 125M and 244M signals, a typical U.S. television studio will have signals from many other sources. At present, these are mainly analog NTSC signals. It is a common practice in producing programming to switch between signal sources during a broadcast. The most common form of switching is between programs and advertisements; however, even within a program, it may be desirable to switch, for example, from a camera source to a computer graphics generator or even to mix computer graphics and live video in a single image.
Due to the tight regulatory requirements for broadcast television signals, it is desirable for signals from various sources to be closely synchronized. Furthermore, color errors can occur if there is a difference in the relative phases of the horizontal sync pulse and the color-burst signal between two video signals which are either switched or mixed. In addition, if signals from two sources are not horizontally aligned so that one signal produces an image with a different center position than the other image, the resulting horizontal shift which occurs when the images are switched is seen as an annoying artifact by the viewer.
With analog signal sources, these adjustments are made by inserting the horizontal synchronization signal into the signal produced by the camera at a desired time relative to the start of the active video portion of the signal. While this same type of adjustment may be made in a digital system, delay adjustments in existing equipment have generally been limited to multiples of a single digital sample.